Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WiFi transceivers, and other communication devices transmit and receive communication signal at various frequencies that correspond to different communication bands and at varying power levels. These devices are typically powered by a portable rechargeable power source, such as a direct current (DC) battery, and rely on a radio frequency (RF) power amplifier to transmit the communication signal. Under some conditions, an RF power amplifier (PA) will draw excessive current from the power supply, such as when subjected to non-optimal (high VSWR) antenna load conditions.
To restrict the current consumed by the power amplifier to remain below circuit destructive levels and to extend system battery life, a current limiting function is typically included in the design of the power amplifier. Integral to the current limiting function is a current sense device. For example, in a global system for mobile communication (GSM) cellular PA module application, the current sense function is typically performed by a specialized, low ohmic surface mount technology (SMT) resistor inserted in series with the PA collector bias inductor. The voltage developed across the SMT resistor is proportional to the PA current and is continuously compared to a reference voltage which represents the desired current limit threshold. Under high current conditions, the voltage across the resistor exceeds the reference voltage and the PA collector bias voltage is reduced to maintain the power amplifier current at the desired current limit threshold.
Unfortunately, the specialized SMT current sensing resistor is costly when compared to the cost of a typical resistor. The combined requirements of low resistance (on the order of 20 milliOhm (mΩ)), tight tolerance (on the order of 5%), low temperature coefficient (thermal coefficient of resistance (TCR) on the order of 200 parts per million (ppm)), and low volume contribute to the high unit cost of the resistor.
Therefore, it would be desirable to maintain the effectiveness of the current sensing function, while reducing the cost of the associated components.